The subject matter disclosed herein relates to a method of heat treating Ni-base superalloys and articles made thereby. More particularly, it relates to a method of heat treating Ni-base superalloys to provide desirable yield strength, ductility and high temperature hold-time crack resistance and articles made thereby.
High temperatures and stresses are normally encountered during operation of jet and land-based turbine engines. The components within these turbine engines must retain high strength under load and other properties at temperatures in excess of 850° F. in order to ensure reliable turbine function over extended periods of operation. Ni-base superalloys have long been recognized as having properties at elevated temperatures that make them desirable for use in critical turbine components that have high operating temperatures, such as turbine wheels, combustors, spacers, blades/vanes and the like. Precipitates of a γ″ are believed to contribute to the superior performance of many of these Ni-base superalloys at high temperatures. Consequently, Ni-base superalloys such as Alloy 706, Alloy 718, Alloy 625 and Alloy 725 have been widely used to form these components in turbines that are used for land-based power generation.
Industrial gas turbine rotors manufactured from Alloy 706 and given the industry standard two-step aging heat treatment in the past have experienced cracking along grain boundaries during operation prior to full life. This problem has been partially addressed by more stringent manufacturing processes including surface processing that induces compressive residual stresses and by manufacturing newer rotors from Alloy 718 or Alloy 706 that has been given a two-step aging heat treatment. However, as the operating temperature and stress requirements for industrial gas turbine and steam turbines are increased, neither Alloy 706, Alloy 718, Alloy 725 or Custom Age 625 PLUS) will satisfy these requirements, and must be replaced by alloys with a better combination of strength, ductility and hold-time crack resistance, while also maintaining excellent corrosion resistance, preferably corrosion resistance at least as high as that of Alloy 706 and Alloy 718, and more preferably at least as high as that of Alloy 725 and Custom Age 625 PLUS.
Therefore, it is desirable to develop Ni-base superalloys that enjoy improved TDCPR, strength and ductility and that also provide excellent corrosion resistance, as well as methods of making such Ni-base superalloys.